1. Field of the Invention
The present invention relates to a CCD image sensing device.
2. Description of the Prior Art
FIG. 1 shows the construction of a CCD image sensing device of a frame intertransfer system (FIT system) having an image sensing region 1 and a storage region 2 arranged along a vertical direction. The image sensing region 1 has sensor elements (light receiving elements) SE consisting of photodiodes, and vertical shift registers 12 combined respectively with vertical rows of the sensor elements SE. Signal charge stored in the sensor elements SE is read simultaneously by the corresponding vertical shift registers 12 in a vertical blanking interval and transferred at a high speed to vertical shift registers 22 in the storage region 2. Then, the signal charge of the sensor elements SE on each vertical row is transferred to a horizontal shift register 3A or 3B in each horizontal blanking interval. The signal charge of the (2n+1)-th (n is 0 and positive integers) sensor elements SE on each horizontal line is transferred to the horizontal shift register 3A as indicated by the arrows of continuous lines, and the signal charge of the 2n-th sensor element SE on each horizontal line is transferred through a transfer gate 4 to the horizontal shift register 3B as indicated by the arrows of broken lines.
The signal charge of the (2n+1)-th sensor elements SE transferred to the horizontal shift register 3A and the signal charge of the 2n-th sensor elements SE transferred to the horizontal shift register 3B in each horizontal blanking interval are transferred sequentially to signal detectors 5A and 5B in synchronism with horizontal scanning speed in the subsequent horizontal scanning interval. Image signals corresponding to the signal charge of the (2n+1)-th sensor elements SE and image signals corresponding to the signal charge of the 2n-th sensor elements SE are sent out in parallel from the signal detectors 5A and 5B, respectively.
Charge (smear charge) stored in the vertical shift registers 12 of the image sensing region 1 and charge stored in the vertical shift registers 22 of the storage region 2 are transferred at a high speed in the vertical direction immediately before the signal change of the sensor elements SE is transferred to the vertical shift registers 22 of the storage region 2, and the charge is swept off through the horizontal shift register 3A, the horizontal transfer gate 4, the horizontal shift register 3B and a smear gate 6 into a smear drain region 7 to suppress smearing.
FIG. 2 is a sectional view taken on line I--I in FIG. 1, showing an electrode construction. The vertical shift register 22, the horizontal shift register 3A, the horizontal transfer gate 4, the horizontal shift register 3B, the smear gate 6, the smear drain region 7 and a channel stop region 8 are arranged in that order on an n-substrate N-Sub.
A vertical transfer pulse .psi.VH is applied to the gate electrode (transfer electrode) 101 of the horizontal shift register 22. A two-phase drive horizontal pulse .psi.H, for instance, is applied to the respective gate electrodes (transfer electrodes) 102 and 104 of the horizontal shift registers 3A and 3B. A horizontal transfer gate pulse .psi.HHG is applied to the gate electrode 103 of the horizontal transfer gate 4. A smear gate pulse .psi.SMG is applied to a smear gate electrode 105.
A voltage SMD is applied to the smear drain region 7, the channel stop region 8 is grounded and a voltage V.sub.sub is applied to an n-substrate N-Sub. FIG. 3 shows the distribution of potential along line A--A in FIG. 2.
A p-well region P-Well corresponding to the horizontal shift registers 3A and 3B are designed so as to be depleted by operating bias voltages for the horizontal shift registers 3A and 3B. Therefore, a hole storage region is created depending on biasing condition and impurity concentration because the smear gate 6, the smear drain region 7 and the horizontal transfer gate 4 isolate the potential of the p-well region P-Well corresponding to the horizontal shift registers 3A and 3B from the ground potential GND.
Holes diffused from the gate electrode 103 and those produced by thermal excitation are collected by potential barriers of the p-well region underlying the gate electrodes 103 and 105 and the holes are collected locally and stored in the p-well region corresponding to the gate electrode 104 as shown in FIG. 4B. The potential of a region storing holes is unstable and affects signal transfer adversely.
At the same time, a hole-depletion region is formed. In the hole-depletion region, holes under the gate electrode 103 is emitted when a positive bias voltage is applied to the gate electrode 103 as indicated by a broken line in FIG. 4A and the potential of the p-well region becomes negative potential when the gate electrode 103 is biased again in negative as indicated by a continuous line in FIG. 4A. Consequently, the potential difference between the gate electrodes 102 and 104 increases to produce dark current due to avalanche.